1. Technical Field
The present invention relates to data processing systems and, in particular, to electrophotographic reproduction devices. Still more particularly, the present invention relates to the adjustment of the timing of a scanning laser beam with respect to a second scanning laser beam in an electrophotographic reproduction device utilizing a plurality of scanning laser beams in order to adjust the timing of a plurality of PEL clocks.
2. Description of the Related Art
Electrophotographic reproduction devices can generally be divided into copiers or printers. The present invention relates primarily to printers.
In an electrophotographic printer, a charged photoconductor is selectively discharged by the operation of a print or imaging station, to provide an electrostatic latent image on the photoconductor's surface. This latent image corresponds to the visual image that is to be printed, first by applying toner to the photoconductor, and then by transferring the toner image to the surface of substrate material such as a sheet of plain paper.
Electrophotographic reproduction devices may be constructed to apply toner to either the photoconductor's discharged area, or to the photoconductor's charged area. The former type of device is called a discharged area developing device, whereas the latter device is called a charged area developing device.
The broad spirit and scope of the invention are not to be limited to a scanning light beam since, as will be appreciated by those of skill in the art, such a scanning beam generally comprises a moving point or spot of light, that can be modulated in intensity--for example, on and off, to form an electrostatic latent image on the photoconductor.
Thus, the term, scanning laser beam, as used herein, is intended to mean any moving point of light to which the photoconductor is sensitive, and which operates to sequentially print the small picture elements, or PELs, of one or more image rows, as the point of light sequentially scans the photoconductor, image row after image row.
In printers of this type, the image to be printed comprises an electronic image signal that may, for example, reside in the page memory of a data processing system. In this page memory, each photoconductor PEL area that is to be discharged may, again by way of example, be represented by a binary "1", in which case each PEL this is to be left in its charged state would be represented by a binary "0". As the spot of light moves across a photoconductor PEL row, the row content of the page memory signal is gated utilizing a PEL clock to control or modulate the spot of light in accurate synchronism with the position of the moving spot of light.
The art has recognized the need to synchronize the gating of the print data signal to laser beam modulator means as a function of the beam's position prior to each photoconductor scan. For example, it is known to use a photosensor that is located on an image plane that is established by the physical location of the photoconductor. As a result of the detection of the laser beam at this start of scan position, a timing pulse, or beam detection indicator, is generated to start the flow of print data to a beam modulator.
Imaging stations used in electrophotographic printers often utilize more than one scanning laser beam. In systems utilizing two beams, for example, each beam scans a different PEL row approximately concurrently so that two PEL rows are simultaneously scanned. Therefore, two beam detection indicators are generated, one for each beam. A fixed time offset between a first pel of a first beam and a first pel of a second beam may be equivalent to an offset of several tens of pels on order to avoid interference between the adjacent beams.
Therefore a need exists for a method and system in an electrophotographic reproduction devices which utilizes a plurality of scanning laser beams for adjusting the timing of a first scanning laser beam with respect to a second scanning laser beam, and therefore the timing of a plurality of PEL clocks, so that the associated PELs are approximately aligned.